1. Field of the Invention
This invention relates to reflector-based antennas, and more particularly to beam shaping of RF feed energy for reflector-based antennas including, but not limited to, single, dual and tri-mode sensors for target tracking.
2. Description of the Related Art
The basic design and operation of reflector-based antennas are well known and well documented in technical literature. In the simplest configuration, one or more RF feed elements are located near the focal point of a reflective surface (e.g. a parabolic dish). The reflective surface acts to collect incoming electromagnetic energy from a distant source in the far field in a particular direction to the feed element(s) in the focal area and/or re-radiate energy from the feed element(s) in a directive fashion towards the same particular direction into the far field. Reflector antennas are used for satellite communication, radio astronomy, target tracking, and many other applications that require a highly directive antenna. One approach for target tracking, commonly referred to as “monopulse tracking”, segments the feed into quadrants with one or more feed elements per quadrant and uses sum and difference configurations of the quadrants to estimate target angular position. As used herein, the term “RF” includes the portions of the electromagnetic spectrum commonly referred to as RF, millimeter wave or microwave.
U.S. Pat. No. 5,214,438 discloses a dual-mode sensor including both a millimeter wave and infrared sensor in a common receiving aperture for target tracking. A selectively coated dichroic element is located in the path of the millimeter wave energy on the axis between the feed and the primary reflector. The dichroic element reflects infrared energy from the primary reflector to a focal point and at the same time transmits and focuses millimeter wave energy. An optical system relays the infrared energy to a focal plane behind the primary mirror. The dichroic element transmits and focuses millimeter wave energy without significant attenuation such that optical and millimeter wave energy may be employed on a common boresight. The IR optical system may increase the central blockage of the RF feed pattern. Tri-mode sensors such as disclosed in U.S. Pat. No. 6,606,066 may position a laser spot tracker forward of the RF feed and transceiver. This laser spot tracker may further increase the size of the central blockage.
U.S. Pat. No. 6,295,034 discloses a feed that includes an array of individual elements, specifically four elements per quadrant, for use in common aperture sensor systems for target tracking. The array elements are configured to increase the overall efficiency of a reflector antenna by flattening the aperture illumination, and also by nullifying the illumination within the centrally blocked portion of the reflector antenna surface. More specifically, the array elements are carefully configured with respect to spacing and excitation, for example, such that the array illuminates only the non-blocked portion of the main reflector. In addition, the array pattern is optimized such that the non-blocked portion of the reflector antenna is quasi-uniformly illuminated. In short, the feed elements are configured to direct a majority of RF energy from the feed towards regions of the main reflector that are not blocked by the dichroic element/IR sensor or laser spot tracker. The carefully configured multi-element feed is cited as providing an increasing in efficiency of about 20% over the conventional monopulse feed (e.g. one element per quadrant).